1. Field of the Invention
This invention relates to a driving-wheel torque control system for automotive vehicles, and more particularly to a driving-wheel torque control system of this kind which has a variable-torque limited slip differential arranged between left and right driving wheels for properly distributing a limited slip differential torque (hereinafter referred to as "LSD torque") to a racing driving wheel and a non-racing driving wheel to thereby control the torque of the driving wheels.
2. Description of the Related Art
Conventionally, a torque distribution control system has been proposed, e.g., by Japanese Provisional Patent Publication (Kokai) No. 64-60433 (hereinafter referred to as "the first prior art system"), which comprises a variable-torque limited slip differential having a multiple disc friction clutch, which is arranged between a pair of left and right driving wheels for generating a limited slip differential torque created by hydraulic pressure externally applied thereto. According to this system, LSD torque is distributed between the left and right driving wheels according to a difference in the rotational speed between the left and right driving wheels so as to properly control relative driving forces of the left and right driving wheels.
If LSD torque is applied upon slippage of one of the left and right driving wheels during cornering of the automotive vehicle, the so-called steering characteristic deviates either in an understeering direction or in an oversteering direction, making it difficult for the vehicle to turn around. To overcome this inconvenience, another limited slip differential torque control system has also been proposed, e.g., by Japanese Provisional Patent Publication (Kokai) No. 3-224827 (hereinafter referred to as "the second prior art system") in which the LSD torque is reduced or cancelled when the two driving wheels are both determined to be in slippage.
In the first and second prior art systems described above, when the vehicle is traveling at a low speed on a so-called split .mu. road having a road surface which has unevenness in friction coefficient from portion to portion, such as a snow-covered road and a bad road, if one of the left and right driving wheels, which is on a low .mu. road portion side, slips, the difference in the rotational speed between the driving wheels is detected, and LSD torque is transmitted to the racing driving wheel on the low .mu. road portion side, and the other or non-racing driving wheel, such that reduced torque is transmitted to the racing driving wheel, and increased torque to the non-racing driving wheel, to limit the difference in the rotational speed between the driving wheels, thereby allowing a proper amount of the driving force of the non-racing driving wheel to be transmitted to the road surface. In this manner, the prior art systems attempt to improve the traveling ability, i.e., the drivability and stability of behavior of the automotive vehicle during running on a split .mu. road.
Further, in the second prior art system, the LSD torque is reduced or canceled upon determining that both the left and right driving wheels are in slippage, based on the difference in the rotational speed between the driving wheels and the difference in the rotational speed between the front and rear wheels, to thereby prevent a so-called sideslip or skid and enhance the traveling stability when the vehicle is running straight.
However, although the traveling ability of the automotive vehicle during traveling on a snow-covered road or a bad road is improved as described above, the first and second prior art systems still have a disadvantage encountered when the vehicle is traveling at a high speed on a pool of liquid on the road. That is, on such an occasion, there occurs a hydroplaning phenomenon such that both the driving wheels race so that the balance in driving force between the two driving wheels is lost to make it impossible to control the vehicle by its steering wheel, resulting in a sideslip or skid of the vehicle. Therefore, the first and second prior art systems cannot prevent degradation in the drivability and traveling behavior of the automotive vehicle in such events.
Further, the second prior art system, which reduces or cancels the LSD torque when it is determined that both the driving wheels are in slippage, has the disadvantage that when the vehicle is traveling on a road having a slippery surface, such as a snow-covered road and an icy road, both the driving wheels are liable to slip, and hence the LSD torque is frequently reduced or canceled, thereby making it impossible to effectively apply the LSD torque, which results in degraded drivability of the vehicle.
Further, when the vehicle is traveling on a so-called split .mu. road having a surface which has unevenness in friction coefficient from portion to portion, a driving wheel on a low .mu. road portion side largely slips, and hence a large LSD torque acts on the driving wheels. Therefore, both the driving wheels suddenly slip, whereupon the LSD torque is suddenly reduced or canceled. As a result, the driving wheel on the low .mu. road portion side slips again, and hence a large LSD torque is generated again, which results in a frequent change in the driving forces of the driving wheels, causing offensive vibrations.
Further, when the vehicle uses studless tires, the vehicle traveling on a snow-covered road exhibits better derivability when the degree of slippage is large provided that the studless tires are new. Therefore, in such cases, the driver of the vehicle may wish to intentionally cause the driving wheels to slip. However, according to the above second prior art system, the LSD torque is reduced or canceled immediately upon detection of slippage of the driving wheels, and hence it is impossible for the driver to obtain drivability of the vehicle as intended.
Further, when the driver intentionally causes the driving wheels to slip during cornering of the vehicle, the LSD torque is also reduced or canceled as described above, and hence the driver cannot expect a behavior of the vehicle as desired.